The present invention relates to a module for bidirectional optical communication capable of performing both transmission and reception used for optical communication or the like. More Particularly, the present invention relates to a module for bidirectional optical communication capable of receiving both wavelength bands of 1.3 xcexcm and 1.55 xcexcm.
In recent years, optical fibers have come to be used for communication. Specifically, 1.3 xcexcm band light is used for communication of information between subscribers and a station, while 1.55 xcexcm band light is used for circuits on optical fiber lines. Home modules are therefore construced such that only signals of the 1.3 xcexcm band are utilized for transmission and reception, as conceptually shown in FIG. 3. That is, referring to FIG. 3, a conventional module includes: a light emitting element 5, such as a semicondutor laser, for emitting transmitting signal light; a light receiving element (photodetector) 52, such as a photodiode, for receiving incoming signal light via a half mirror 53; a converging lens 54 for coupling the transmitting (outgoing) signal light reflected by the half mirror 53 to an optical transmission line 55 such as optical fibers; the optical transmission line 55 for transmitting converged light; and a branching filter 56 which transmits nearly 100% of 1.3 xcexcm band light and reflects nearly 100% of 1.55 xcexcm band light. This structure allows transmitting signal light emitted by the light emitting element 51 to be reflected by the half mirror 53, and to be incident on the optical transmission line 55 and finally received by a recipient. In the case of receiving a signal from a sender, the branching filter 56 reflects substantially all of 1.55 xcexcm band light sent from the optical transmission line 55, while transmitting substantially all of 1.3 xcexcm band light. The transmitted incoming signal light then passes through the half mirror 53 to be received by the light receiving element 52 where it is converted to an electric signal. In this way, optical communication is realized. The transmission and reception are alternately switched by time division to avoid interference therebetween.
The conventional module for optical communication is constructed to transmit and receive only 1.3 xcexcm band light as described above. However, a recent trend is that another frequency band of 1.65 xcexcm is used for testing and the 1.55 xcexcm band is used for CATV (community antenna television) communication, to be implied that the 1.55 xcexcm band is to be utilized at home. It is therefore desirable for home modules to have an ability of receiving 1.55 xcexcm band light in addition to 1.3 xcexcm band light. To realize this ability, a construction as shown in FIG. 4 is required, where a module 60 is added to the construction shown in FIG. 3 so that the 1.55 xcexcm band light reflected by the branching filter 56 is reflected again by a total reflection mirror 61 to be received by a light receiving element 62 for the 1.55 xcexcm band. This increases the size of the entire module, as well as increasing the cost thereof.
In view of these circumstances, it is an object of the present invention to provide a single-packaged simple and inexpensive module for bidirectional optical communication capable of transmitting/receiving a 1.3 xcexcm band signal and receiving a 1.55 xcexcm band signal.
The module for bidirectional optical communication according to the present invention includes: a light emitting element for emitting transmitting signal light of a first frequency band and directing the light to an optical transmission line; a light receiving element having first and second light receiving portions for receiving respectively receiving (incoming) signal light of the first frequency band and that of a second frequency band which are sent from the optical transmission line; and an optical branching filter interposed between, the light emitting element and the light receiving element, and the optical transmission line, wherein the optical branching filter comprises a transparent member having a uniform thickness and a refractive index greater than that of the air, a first dielectric film which transmits substantially 50% of light of the first frequency band while reflecting substantially 50% of light of the first frequency band and transmits substantially 100% of light of the second frequency band being formed on a first portion of a first surface of the transparent member corresponding to an incident portion of the transmitting signal light emitted from the light emitting element, a second dielectric film which transmits substantially 100% of light of the first frequency band and reflects substantially 100% of light of the second frequency band being formed on a second portion of an opposite second surface of the transparent member at which the receiving signal light incident on the first portion from the optical transmission line and passing through the transparent member arrives, a third dielectric film which reflects substantially 100% of light of the second frequency band being formed on a third portion of the first surface of the transparent member at which the light of the second frequency band reflected by the second portion arrives, a fourth dielectric film which transmits substantially 100% of light of the second frequency band being formed on a fourth portion of the second surface of the transparent member at which the light of the second frequency band reflected by the third portion arrives, and the first and second light receiving portions of the light receiving element are arranged so as to receive the light of the first frequency band output from the second portion of the transparent member and the light of the second frequency band output from the fourth portion of the transparent member, respectively.
In one aspect, a reflector is provided for reflecting light which is incident on the first portion of the transparent member from the light emitting element and pass through the transparent member, not being reflected, toward the first light receiving portion of the light receiving element. Due to the time-division switching of transmission and reception, the existing light receiving element can be used for monitoring the power of the light emitting element, eliminating the necessity of an additional light receiving element for automatic power control (APC).
The fourth dielectric film may be formed so as to have the same transmittance and reflectance as the first dielectric film and the third dielectric film may be formed so as to have the same transmittance and reflectance as the second dielectric film. This simplifies the film-formation process, and also eliminates the necessity of distinguishing one surface from the other during assembly by setting the film-formation portions of the opposite surfaces at positions symmetrical to each other with respect to a center point.
The third dielectric film may be a total reflection film, and/or the fourth dielectric film may be a transparent film or no fourth dielectric film may be formed. With these constructions, one surface can be distinguished from the other at a glance, simplifying the work in the assembly process.
The light receiving element is preferably composed of a single chip semiconductor device having the first light receiving portion and the second light receiving portion. This allows for compact construction.
A face of the light emitting element is preferably arranged to be tilted from a 90xc2x0 position with respect to an optical path of the receiving signal light output from the optical transmission line and reflected by the optical branching filter. And a face of the light receiving element is preferably arranged to be tilted from a 90xc2x0 position with respect to an optical path of the receiving signal light output from the optical transmission line and transmitted by the optical branching filter. This prevents the light from the optical transmission line from returning to the optical transmission line by being reflected by the face of the light emitting element or the light receiving element, avoiding generation of noise.